TITLE:
The Evaluation of Hydrogel Rectal Spacer Movement and Absorbance during the Treatment of Prostate Cancer with Proton Therapy
AUTHORS:
Alexis Reinders, Elisheba Odei, Bosco Giap, Breana Nguyen, Abby Grace Lin, Huan Giap
KEYWORDS:
Rectal Spacer, Prostate Cancer, Proton Therapy, Hydrogel, SpaceOAR
JOURNAL NAME:
International Journal of Medical Physics, Clinical Engineering and Radiation Oncology,
Vol.15 No.3,
July
30,
2026
ABSTRACT: Background: Hydrogel perirectal spacers are widely used to reduce rectal toxicity during prostate radiation therapy; however, their stability and degradation specifically during proton therapy remain poorly characterized. This study evaluated hydrogel spacer geometry and absorption throughout a course of intensity-modulated proton therapy (IMPT). Methods: Ten patients with localized or recurrent prostate cancer underwent hydrogel spacer placement before IMPT. Eight patients received SpaceOAR. All patients were treated to 70 Gy in 28 fractions using two lateral beams with single-field optimization. Baseline CT and contrast-enhanced MRI were obtained for radiation planning, followed by bi-weekly adaptive CT and MRI scans without contrast. Spacer thickness was measured at the prostate base, mid-gland, and apex on serial MRIs. Changes in spacer size were analyzed per patient and in aggregate. Results: Interpatient variability exceeded intrapatient changes over the treatment course. One patient demonstrated complete spacer disappearance by the third assessment, requiring rectal balloon placement. Three patients exhibited notable reductions in spacer size (18%, 33%, and 72% across all regions). The remaining patients showed a